1. Field of the Invention
This invention relates to optical systems comprising two elements suitable for use in photographic devices having cylindrical image surfaces.
2. Description Relative to the Prior Art
A two-element camera objective lens system with a curved image surface was disclosed in U.S. Pat. No. 1,082,678. The lens system therein disclosed worked with "a flexible film used for making the negative to conform more accurately to a focal surface of the lens, with consequent improvement in the definition of the image." According to the inventor of U.S. Pat. No. 1,082,678, the curved image surface was largely introduced to compensate for "spherical aberration". The inventor did not teach how to design a lens that is best suited for imaging an object onto a curved image surface. Instead, the inventor explicitly states that the design of a well-corrected system consisting of two or more elements is a very difficult task and it is to avoid doing so that he has made his invention.
U.S. Pat. No. 3,006,248 by W. R. Linke (issued in 1961) discloses an objective for a camera having a cylindrical image surface and comprises two meniscus lenses. The disclosed lens system, "while of the symmetrical type is not completely symmetrical". It has a rear landscape lens with a meniscus corrector lens in front. The power of the front lens is weaker than the power of the second lens. This lens system is corrected for lateral color as in a periscopic lens; is corrected for coma in a similar fashion; and has astigmatism carefully divided about the curved image surface. Because the objective in the last mentioned design has little distortion (when analyzed with a plano image plane), it does not compensate for distortion introduced by the cylindrical image surface. Because distortion is introduced by the cylindrical image surface, the overall system suffers from distortion.
Proposals have been made for the systems including two different asymmetrically located lenses. For example, U.S. Pat. No. 55,196 (J. Zentmayer, 1866) disclosed a doublet made of meniscus lenses of different curvatures, its exterior surfaces concentric, arranged around an aperture stop. The photographic system disclosed in U.S. Pat. No. 55,196 was designed to have a wide field of view and utilized a planar (i.e., uncurved) focal plane. The draw-back of this system, according to Zentmayer is that "The lenses must of necessity be thin." FIGS. 1 through 3 of U.S. Pat. No. 55,196 show lenses that look like thin shells. Such lenses are generally difficult to make with conventional equipment, thus making them very expensive. In addition this lens is not intended for use with a curved image surface.
In his book, Lens Design Fundamentals, Dr. R. Kingslake teaches how to design a periscopic lens. This lens system is obtained by placing two identical landscape lenses symmetrically about a central stop. The periscopic lens has little distortion, lateral color (sometimes referred to as a lateral chromatic aberration) and coma, but has spherical aberration, astigmatism and petzval field curvature. This lens is intended for use with a flat image plane because the tangential field curvature is nearly flat. Such a lens system will have very small distortion because it is symmetrical. If the film surface is curved then the resulting picture will have large distortion. An example of a periscopic lens is disclosed in U.S. Pat. No. 2,586,418 (J. K. Davis, 1952).